Children as young as seven are purposely harming themselves, according to a report in the medical journal "Pediatrics." It is the first formal investigation of self-harm in under-12s.

Academics studied 665 schoolchildren in the Denver and New Jersey areas across three age groups: third, sixth and ninth grades. Overall, 8 percent of respondents admitted to self-harming, and the figure was only slightly lower -- 7.5 percent -- for the third-grade group, which included children aged 7.

"A lot of people tend to think that school-aged children, they're happy," says lead researcher Benjamin Hankin, an associate psychology professor at the University of Denver. "It's unfortunately probably more common than we want to think."

Of self-harming third-graders, the figures were similar for boys and girls. The most common method was hitting themselves -- which accounted for 60 percent, with just a few cases of cutting or burning. Across the age groups, girls were around 50 percent more likely to self-harm and of these 64 percent cut or carved their skin. That is five times higher than for boys, for whom the most common method was hitting themselves, which accounted for 55 percent.

"The results are not surprising because there is a growing trend of younger people self-harming," child mental health specialist, psychologist and Middlesex University lecturer Dr. Fiona Starr told Metro. "We are also seeing with the rise of eating disorders that pressure on young people is affecting them, either from their peers or popular culture."

Dr. Starr added that self-harming can be "extremely addictive" and that targeted campaigns in schools are needed to prevent the problem from worsening.